External Morphology of the Erect Glans Penis of the Dog ' BENJAMIN L. HART2 AND RALPH L. KITCHELL3 Department of Veterinary A n a t o m y , University of Minnesota, S t . Paul, Minnesota ABSTRACT The external morphology of the erect glans penis was studied and compared to that of the nonerect organ. Morphological changes that occur with tumescence and detumescence were also reported. Some distinct parts of the erect pars longa glandis which are not distinguished i n classical nomenclature are described and labeled. These structures are the urethral process, corona glandis and collum glandis. These distinct morphological areas coincide with areas which seem to be functionally distinct in both the erect and nonerect glans penis. The implications that these observations may have for considerations of comparative anatomy of the external male genitalia are discussed The external morphology of the glans penis of mammals varies greatly from species to species. In these mammals the anatomical description of structures such as the urethral process, corona glandis and collum glandis is based on the appearance of the nonerect or flaccid penis. We became interested in comparing the morphology of the nonerect penis to the erect penis as a result of previous studies on spinal sexual reflexes elicited by genital stimulation (Hart, '64). Initially it was assumed that stimulation of any area of the glans would have about the same effect as any other on the reflexes observed. It soon became apparent, however, that stimulation of one area of the glans had effects entirely different from stimulation of another area (even in the nonerect penis). After repeated observation of these effects, it was also noted that, in the erect glans penis, different structures were visibly discernible which were not apparent in the nonerect state and had not been described previously (Ellenberger and Baum, '43; Miller, '52; Miller, Christensen and Evans, '64; Sisson and Grossman, '53; Bradley, '59; Nickel, Shummer and Seiferle, '60). These areas, which were grossly visible as distinct structures, were the same areas which, when stimulated, resulted in different functional responses. It is the purpose of this paper to first compare the erect penis of the dog to the nonerect organ in regard to external morphology and, secondly, to discuss the implications of ANAT. REC.,152: 193-198. these findings for classical anatomical nomenclature. MATERIALS AND METHODS The experimental subjects were adult mongrel male dogs. The progression of erection was studied visually and recorded cinematographically. Two types of experimental procedures were used. In one group of 15 animals, erection was elicited by use of an artificial vagina similar to the one described by Harrop ('54). When a full erection had been obtained, the vagina was removed. In the second group of animals, totaling 16 ( 4 were previously members of the first group), a spinal transection was performed, and the animals were maintained in the chronic spinal state from 40 to 185 days. The procedure for the spinal transection and postoperative care is presented elsewhere (Hart, '64). In the chronic spinal preparation, an erection was easier to elicit without the use of an artificial vagina, by merely holding the penis behind the bulbus glandis and rubbing the area surrounding the external urethral orifice. The progression and subsidence of erection 1 This investigation was supported in part by Public Health Service Training Grant 5T1-GM-386-04 from the Division of General Medical Sciences. Based i n part on a thesis submitted by the senior author to the Graduate School of the University of Minnesota, i n partial fulfillment of the requirements for the Ph.D. degree. 2 Present address: Department of ,Anatomy, University of California, Davis, Califomla. 3 Present address: College of Veterinary Medicine, Kansas State University, Manhattan, Kansas. 193 194 BENJAMIN L. HART AND RALPH L. KITCHELL was observed and recorded cinematographically in this second group. Brief review of anatomy of the canine penis In order to facilitate discussion of the results, the anatomy of the penis will be briefly reviewed. The canine penis consists of a root, body and glans. The root is a paired structure composed of two crura which arise from the ischial tuberosities of the pelvic girdle. The body is the portion of the penis extending from the union of the two crura to the glans. The glans or distal portion is quite long in the dog and is divided into a caudal or proximal bulbous part, the bulbus glandis and a cranial or distal part, the pars longa glandis. Since the prepuce is reflected onto the glans about halfway between the two extremities of the bulbus, the glans is the only portion of the penis that can ordinarily be exposed by pushing back the sheath. The rest of the penis can be easily palpated, however. In the center of the penis is a bone (0s penis) extending from the distal portion of the body to almost the tip of the glans. When the penis is not erect this bone helps keep the organ relatively rigid. The most extensive erectile tissue of the penis is the corpus cavernosum urethrae (spongiosum). This structure surrounds the urethra within the pelvic cavity and extends in this relationship throughout the entire length of the penis. Just proximal to the entrance of the pelvic portion of the corpus ca,vernosum urethrae into the body of the penis, this cavernous body gives off two bulbous expansions which project proximally between the two crura of the root. These are the so-called bulbs of the urethra. The bulbus glandis is a cavernous dorsal expansion of the corpus cavernosum urethrae. The erectile tissue of the crura of the root is the paired corpus cavernosum penis. This is the least expansive of the erectile bodies of the penis. The erectile tissue of the pars longa glandis is separated from the erectile tissue of the bulbus glandis by a fibrous sheath, but it is similar in structure and expansive characteristics. The extrinsic muscles of the penis are the striated bulbocavernosus, ischiocavernosus and ischiourethral muscles. The canine penis also contains a retractor penis muscle. The retractor is composed mostly of smooth muscle fibers and extends from the first and second coccygeal vertebrae to insert on the penis near the fornix of the prepuce. The bulbocavernosus muscle arises from the external anal sphincter. The ischiocavernosus muscle originates from the ischial tuberosity and inserts on the corpus cavernosum penis. The ischiourethral muscle is paired and also originates on the ischial tuberosity adjacent to and cranio-dorsal to the origin of the ischiocavernosus muscle. The insertion of the ischiourethral muscle is onto a fibrous ring encircling the common trunk of the left and right dorsal veins of the penis. The glans penis of the dog, according to classical gross description and terminology, is divisible only into a pars longa glandis and bulbus glandis. No other structures, such as a n urethral process, corona or collum, some of which are described for other species such a s man, the stallion, the bull and the ram, have been labeled. The reason for this undoubtedly is because the penis in the erect state has not been carefully studied. RESULTS The penis of one of the dogs as erection progressed from the nonerect state to complete erection is shown in figure 1. It was clear that as erection progressed certain structures of the pars longa glandis became quite prominent. Though the nonerect glans has essentially the same appearance in all of the dogs observed, there is considerable variation in external morphology among the animals. The ranges of differences in the erect state observed is shown in the four specimens presented in figure 2. During functional studies (Hart, '64) stimulation of these morphologically distinct regions of the erect penis evoked distinctly different responses in both the intact and the spinal preparation. Since these studies necessitated repeated reference to the respective parts, some new terms have been introduced to describe these regions. The terminology used is in keeping with what would probably be analogous parts in other domestic species (fig. 3). At the free extremity of the erect pars longa Fig. 1 Photographs to illustrate morphological changes during the progression of erection i n the glsns penis. The prepuce is held behind the bulbus glandis in all pictures. ( A ) Nonerect (holding the sheath back has forced a protrusion of the epithelium over the 0 s penis at the free end of the pars longa glandis); ( B ) and ( C ) stages intermediate between the nonerect and the completely erect glans; ( D ) complete erection. 195 Fig. 2 Photographs to illustrate variations in morphology in the erect glans penis of the dogs studied. All of the animals were mongrels so no specific references could be made to breed. 196 MORPHOLOGY O F THE GLANS PENIS glandis an “urethral process” is quite apparent. At the apex of the process is the external urethral orifice. Just caudal to the urethral process is a distinct corona, and hence this will be termed the “corona glandis.” Both the urethral process and the corona are, in the erect glans, deep scarlet in color and have a wrinkled surface (figs. 1 and 2). The portion of the pars longa glandis extending from the corona to the bulbus glandis will be called the “collum glandis.” The collum is turgid and usually has a bluish-white appearance. The appearance of these parts of the pars longa glandis during erection and subsidence of erection was also studied. The mechanism of erection with regard to en- gorgement of the cavernous spaces is reviewed and described by Christensen (’54). However, since the terminology introduced above was not used by Christensen, changes in these structures were not described by him. The erectile process was divided into four stages to facilitate description of erection. The first change at the commencement of erection is an engorgement of cavernous spaces in the crura of the penis. This engorgement does not increase the diameter of the penis to any noticeable extent; it serves mostly to stiffen the organ. The next region to become engorged is the bulbus glandis. When the bulbus has become markedly engorged, “stage one” has Nonerect S t a a e one S t a g e two S t a a e three Stage four Subsiding Corona glandis Collum glandis Fornix \ Urethral process L Pars longo glandis 197 Bul bus glandis Fig. 3 Drawings made by projecting frames taken from a motion picture of the progression and subsidence of erection. The prepuce has been reflected proximally to expose the bulbus glandis. “Stage four” is used as the reference in labeling the various parts of the glans penis. Erection was elicited by holding the penis caudal to the bulbus glandis and rubbing the urethral process. 198 BENJAMIN L. HART AND RALPH L. KITCHELL been attained. In some cases, the bulbus glandis may increase in size to the extent that it reaches almost maximum distention before the pars longa glandis starts to enlarge. When the pars longa glandis has increased noticeably in diameter and the urethral process has become extended, “stage two” has been reached. Since “stage four” is defined as complete erection, “stage three” is taken as the interval between the second and fourth stages. During the interval of “stage three,” the glans continues to increase in diameter, and the corona glandis becomes prominent. In “stage four” the urethral process and corona are at their maximum distention although the process does not appear to be as prominent as in the preceding stage. This is because, during the “third stage,” the corona has become disproportionately more engorged than the process, and the corona has encroached upon the caudal aspect of the process. With complete erection, the contrast between the scarlet red urethral process and corona and the bluish-white collum and bulbus is most apparent (figs. 2 and 3). Subsidence of erection is not the converse of tumescence. The engorged bulbus subsides markedly before detumescence begins in the pars longa glandis. As erection continues to subside, the corona and urethral process become especially prominent because the detumescence of the pars longa glandis takes place first in the collum glandis (fig. 3 ) . DISCUSSION The terms used to label the parts of the penis are those which are used to label what are probably comparable structures in other species. The glans penis of the horse, for example, possesses an urethral process, a corona and a collum in somewhat the same topographical arrangement. However, a troublesome point in this analogy is the existence of a prominent bulbus glandis in the canine species more proximal than the collum glandis. None of the other common domestic species have a bulbus glandis. In man the collum glandis or neck appears to be the most proximal part of the glans (Markee, ’53; Grant, ’62). Since the horse does have a processus dorsalis glandis located more proximally on the penis than the collum, it does seem appropriate to use the terminology suggested. The results of this work raise a question of general anatomical description and labeling. When a structure becomes extremely modified in one physiological state to the extent that distinct structures become evident which are hardly discernible in another state, upon which of the two states should naming and classification of structures be based? The reason, obviously, that the urethral process and corona have not been described for the dog is that little work has been done on the erect organ from a functional and morphological standpoint. The point is once again to be emphasized that the study of anatomy on the cadaver is often misleading and that, when functional aspects are considered, the classical species differences may be more superficial than erstwhile believed. LITERATURE CITED Bradley, 0. C. 1959 Topographical Anatomy of the Dog, 6th Ed. Oliver and Boyd, Edinburgh. Christensen, G. C. 1954 Angioarchitecture of the canine penis and the process of erection. Am. J. Anat., 95: 227-261. Ellenberger, W., and H. Baum 1943 Handbuch der Vergleichenden Anatomie der Haustiere. Revised by 0. Zeitzschmann, E. Ackerknect and H. Grau. Springer, Berlin. Grant, J. C. B. 1962 An Atlas of Anatomy, 5th Ed. Williams and Wilkins, Batlimore. Harrop, A. E. 1954 A new type of canine artificial vagina. Brit. Vet. J., 110: 194-195. Hart, B. L. 1964 Spinal sexual reflexes in the male dog and their relation to sexual behavior. Ph.D. Thesis, University of Minnesota. Markee, J. E. 1953 The Urogenital System. In: Morris’ Human Anatomy, 11th Ed., J. P. Schaeffer, Ed. pp. 1493-1569. McGraw-Hill, New York. Miller, M. E. 1952 Guide to the Dissection of the Dog, 3rd Ed. Published by the author, Ithaca, New York. Miller, M. E., G. C. Christensen and H. E. Evans 1964 Anatomy of the Dog. W. B. Saunders, Philadelphia. Nickel, R., A. Schummer and E. Seiferle 1965 Lehrbuch der Anatomie der Haustiere. Paul Parey, Berlin and Hamburg. Sisson, S., and J. D. Grossman 1953 The Anatomy of the Domestic Animals, 4th Ed. W. B. Saunders, Philadelphia.